1. Field of the Invention
This invention relates to a cooling system for internal combustion engines used in trucks and other motor vehicles and, in particular, to a cooling system utilizing a charge air cooler and an exhaust gas cooler in combination with a radiator.
2. Description of Related Art
Stricter emissions requirements have forced the use of partial exhaust gas recirculation as a means of achieving more complete combustion, and this has necessitated the cooling of the recirculated exhaust gas before introducing it into the engine intake manifold. FIG. 1 shows a typical heavy duty truck cooling system having a liquid-cooled exhaust gas recirculation (EGR) cooler. The engine cooling system comprises an internal combustion engine 20 utilizing conventional liquid engine coolant. The liquid coolant heated by operation of the engine exits the engine through line or hose 61 and passes through a thermostat 30. If the coolant is below the thermostat set temperature it is passed through line 63 to coolant pump 32 and back through line 65 to the engine. If the coolant is above the thermostat set temperature, it is sent through line 62 to otherwise conventional air cooled radiator 22 where ambient air flow 60, 60a and 60b passes through the radiator by means of a fan (not shown) as well as movement of the vehicle in which the engine is mounted. The cooled liquid coolant then passes through lines 57 and 59 back to the coolant pump before returning to the engine.
For mixture with the fuel, the engine utilizes inlet air 40 that passes through a filter (not shown) and is compressed by a turbo- or supercharger. The engine system depicted herein utilizes engine exhaust gases exiting through lines 50 and 54 in a turbocharger in which turbine 26 drives compressor 28. After passing through the turbine blades, the exhaust gas exits through line 55 to the exhaust system (not shown). After compression, the charge air passes through line 42 to air-to-air charge air cooler (CAC) 24 mounted upstream of radiator 22. The cooled charge air then exits CAC 24 through line 44.
A portion of the exhaust gas exiting through line 50 passes through line 52 and through an EGR valve 48. The exhaust gas then passes through line 56 to EGR cooler 34, which is a liquid-to-air heat exchanger that cools the hot exhaust gases using the cooled liquid engine coolant entering through line 57. Because brazed aluminum heat exchanger construction is not capable of withstanding the high exhaust gas temperatures, typically, such an EGR cooler must be of high-temperature heat exchanger construction; that is, made of materials able to withstand higher temperatures than brazed aluminum, such as brazed stainless steel, brazed cupro-nickel, brazed copper, and the like. The cooled recirculated exhaust gas then exits the EGR cooler through line 58, where it mixes with the cooled charge air from line 44. The mixture of cooled recirculated exhaust gas and charge air then proceeds through line 46 to the intake manifold 21 of engine 20 for mixture with the fuel and then to the engine combustion chambers.
The system has two disadvantages: 1) the high cost of stainless steel or other high temperature EGR cooler construction and 2) the cooling limitation resulting from the use of engine coolant at approximately 180° F.
FIG. 2 shows another prior art heavy duty truck cooling system in which the exhaust gas which is to be recirculated is mixed with the hot charge air coming from the turbocharger for cooling in an air-cooled heat exchanger. Since the liquid engine coolant does not need to cool the exhaust gas, the liquid engine coolant passes through line 57 from radiator 22 and back to coolant pump 32 for return to the engine. The hot exhaust gas exiting EGR valve 48 passes through line 56 where it combines and mixes with compressed, heated charge air in line 41 exiting compressor 28. The combined heated exhaust gas and charge air then passes through line 43 to a brazed stainless steel combination exhaust gas recirculation and charge air cooler 24′ upstream of radiator 22. Alternatively, the combination exhaust gas recirculation and charge air cooler may be made of other high temperature construction such as the aforementioned brazed cupro-nickel or brazed copper. After the charge air and exhaust gas are cooled by ambient air 60 passing through CAC 24′, the cooled combined exhaust gas and charge air then pass through line 45 to engine intake manifold 21. This approach does allow the recirculated exhaust gas and charge air to be cooled to a temperature close to that of the ambient cooling air, which will always be much less than that of the engine coolant. However, it does not solve the expense problem related to high temperature-resistant construction and, in fact, increases the expense by requiring stainless steel or other expensive high temperature material to be used in a very large combination EGR/CAC.
In addition to having high material costs, prior systems and methods of cooling charge air and/or recirculated exhaust gases in an internal combustion engine have not been able to individually tailor thermal performance of individual heat exchanger units in a space-saving package.